1. Field of the Invention
The present invention relates to a process for preparation of molding resins. More particularly, the invention relates to a process for preparation of molding resins comprising chain-extending polybutylene terephthalate (hereinafter referred to as "PBT") with an isocyanate prepolymer in the presence of a reinforcing filler. In order to prepare the molding resin according to the present invention, it is preferred that PBT be kneaded and reacted with an isocyanate prepolymer in the presence of a reinforcing agent in an extruder to accomplish formation of molding pellets simultaneously with the chain-extending reaction.
2. Description of the Prior Art
It is known that PBT has many excellent properties and is especially excellent in moldability and that PBT has a very high industrial value. However, when PBT is used as an industrial material, physical and chemical properties required in the final uses are not completely sufficient and therefore, PBT must be modified when it is practically used as an industrial material.
It is considered that the following three methods may be adopted for modification of PBT:
(1) mechanical blending of PBT with other polymer or additive. PA1 (2) Copolymerization or copolycondensation. PA1 (3) Block copolymerization.
In the mechanical blending method (1), there is included a problem concerning the compatibility of the polymer or additive with PBT, and mechanical and other properties are often degraded drastically by incorporation of other polymer or additive. Further, the copolymerization or copolycondensation method (2) is defective in that the component to be copolymerized or copolycondensed is limited to specific glycols and diamines and undesirable results such as reduction of the crystallinity and moldability of PBT are often brought about by disturbance of the polymer chain arrangement caused by the copolymerized or copolycondensed component. The block copolymerization method (3) does not cause drastic reduction of mechanical properties or reduction of the crystallinity and moldability inherent of PBT, and it is deemed that this method will be effective. However, no satisfactory technical means for performing this block copolymerization has been developed or proposed.
Furthermore, in preparing polyesters such as PBT according to the melt polycondensation method, if it is intended to obtain a product having a high molecular weight, since the melt viscosity of the polymer is extremely increased, a special apparatus must be used for a highly viscous resin and a high vacuum must be maintained in the reaction system at the final stage of the polycondensation process. Thus, many technical problems are left unsolved.
As means for solving the foregoing problems, we already proposed a process for preparing molding resins by chain-extending low-molecular-weight PBT having hydroxyl groups on both the chain ends with a diisocyanate in the presence of a reinforcer (see Japanese Patent Application Public Disclosure No. 99741/1974). In this method, since the reaction between low-molecular-weight PBT and diisocyanate is not uniformly advanced, a homogeneous reaction product cannot be obtained. More specifically, in addition to the intended reaction between the diisocyanate and low-molecular-weight PBT, there are caused such side reaction as the mutual reaction between diisocyanate molecules and the reaction of the diisocyanate with the resulting urethane group, and it has been confirmed that the obtained product is inferior in heat resistance.